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Assuming here might be someone who knows something about this, I wanted to ask what is behind this round structure I have spotted today on Google Earth:

Google Earth map

There seems to be a large (~200 km), nearly perfect half-circle covering the states of Alabama, Mississippi and Tennessee. How could this regular structure possibly originate? I did some research on the web, but could not find anything. It looks a bit like an impact crater, but there is none listed in this location and especially of this large size. So how did this structure emerge?

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migrated from gis.stackexchange.com Jan 13 '16 at 20:29

This question came from our site for cartographers, geographers and GIS professionals.

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This is a sedimentary sequence representing the shoreline of a Cretaceous-Paleogene inland sea, the Western Interior Seaway. You can look at the sequence of sediments laid down in the USGS Geological Map of North America. I recommend downloading the Southern Sheet in high resolution and the Explanation Sheet to explain what's going on.

The land use pattern as seen by other people answering this question is actually putting the effect in front of the cause; due to the nature of these sediments being a positive influence on the fertility of the land, it is more likely to be used for farming.

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  • $\begingroup$ Okay, thank you. So this is a prehistoric shoreline. But is there any reason for it to be so perfectly circular? Or did that just happen by chance? $\endgroup$ – Adrian Jan 14 '16 at 13:00
  • $\begingroup$ It's a very hard question to answer. We can look at the sedimentary sequence from road cuttings and drill core, and determine lots of things such as paleocurrent flow direction and the environment of deposition (tidal flat, beach, shallow ocean etc) and piece together a vague description of what was going on at the time, but in the end it's a 60-odd million year old shoreline. Looking at the geology map it seems like there might be a headland of crystalline basement that the curve may have formed around, but there's probably a Masters project in geomorphology to be done to answer this. $\endgroup$ – Ben MS Jan 14 '16 at 21:36
  • $\begingroup$ Well I thought you had some direct information about the origin of this shape. Thank you for your answers. $\endgroup$ – Adrian Jan 15 '16 at 10:35
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It is indeed a land use pattern, but I would guess it is driven by geology. I'm sure someone will come up with some details, but in the meantime check this wikipedia on the geology of the Appalachians.

See how this map corresponds to what you're seeing on the landscape.

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The clue is its location at the southwestern end of the Appalachians. Compressive mountains have to terminate somewhere, somehow. In this case the differential strain energy between the Appalachians and the Alabama-Mississippi sedimentary trough has been taken up by deformation of the softer younger sediments. Browse through mountain chain terminations on Google Earth, and you may find other examples (e.g. Carpathians, north east Iran) although your example is one of the clearest.

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